1. Field of Invention
The present invention relates to a laser marking device for describing geometric forms or figures on a surface that is being irradiated with laser light using two-dimensional scanning.
2. Description of Prior Art
Laser marking devices are known that apply laser light onto the surface of an object so that it is worked for the description of geometric figures and characters. An example of such laser marking device is shown in FIG. 5, which comprises a CO.sub.2 laser 28 as a laser source, an X-axis scanner 26 and a Y-axis scanner 27, allowing the laser light from the CO.sub.2 laser 28 to be deflected two-dimensionally, and a computer system 20 for controlling both the ON/OFF operation of the CO.sub.2 laser 28 and the deflecting operation of the two scanners 26 and 27.
A CPU 21 in the computer system 20 executes the computer program stored in a ROM 22 and constructs coordinate data to be supplied to the X-axis scanner 26 and the Y-axis scanner 27, and supplies a laser ON/OFF control signal to be supplied to the CO.sub.2 laser 28.
One way to construct the coordinate data is described below with reference to an exemplary case of describing a geometric form that comprises a straight line element A-B and a circular arc line element B-C as shown in FIG. 6. A circle is dealt with as a special case of an ellipse where its major and minor axes coincide with each other. ROM 22 registers not only the coordinates (X.sub.1, Y.sub.1) for the start point of line element A-B, (X.sub.2, Y.sub.2) for the end point of line element A-B or for the start point of line element B-C, (X.sub.3, Y.sub.3) for the end point of line element B-C, the coordinates (X.sub.4, Y.sub.4) for the center of the ellipse and R as the length of the major axis or minor axis of the ellipse, but also the kind of the line each element forms a part (i.e., whether it is a straight line, an ellipse or the like). Coordinate data for a number of points on the straight line or the arc of a circle that connect the start and end points of each graphic element are computed by software-based arithmetic operations.
Another way to construct the coordinate data is to compute preliminarily the coordinate data for a number of points that constitute each graphic element, register them in a memory like ROM 22, and retrieve them from the memory when describing a geometric form.
However, the conventional laser marking devices have had the following problems.
The first problem concerns the computation of coordinate data. The first of the two methods described above has had the disadvantage of taking an unduly long time to perform the software-based arithmetic operations. Further, according to this method, the X-axis scanner 26 and the Y-axis scanner 27 are activated upon receipt of the coordinate data for the points of locus for one graphic element. After the description of that line element is completed, the coordinate data for the next line element are computed and subsequently sent to the X-axis scanner 26 and the Y-axis scanner 27. Both the X-axis scanner 26 and the Y-axis scanner 27 will remain inactive until after the description of the next line element is started and during this inactive period, the laser light will continually be applied on to the start point of the next line element. As a result, the start point of the subsequent graphical element is subjected to a greater degree of laser working than any other points of locus, resulting in nonuniform description of the geometric form.
Consider, for example, the case of describing the geometric form shown in FIG. 6. Since a comparatively long time is taken to compute the coordinate data on the points of locus over the circular arc B-C, laser light scanning will stop at point B after the description of straight line A-B before the description of the circular arc B-C starts in the subsequent step.
As the scanners 26 and 27 come to a stop and then start again, the scanning speed will be decelerated and accelerated. In either period, the amount of laser light will increase to cause a problem of the same nature as described in the preceding paragraphs.
The problem caused by the first method in connection with the computation of coordinate data is lacking in the second approach but, on the other hand, a memory is necessary that has a large enough capacity to store all of the coordinate data on a number of points of locus that comprise each of the line elements to be described.
The second problem to be discussed concerns the deflection scanning with laser light. Since each X-axis scanner 26 and the Y-axis scanner 27 each has scanning optics such as a galvano-mirror having an inertial force, the scanners 26 and 27 will not respond quickly enough to a scan start command. Rather a delay time will occur as determined by the inertial force or the like of the scanning optics. In contrast, the laser source such as a CO.sub.2 laser has practically no delay time since it starts to oscillate and emits laser light almost simultaneously with the application of an ON command.
FIG. 11A illustrates how the response of the scanner to a command thereto delays by a given time T when describing a straight line of length L. In the illustrated case, a laser ON command is issued as soon as a scan start command is supplied to the scanners, whereupon the emission of laser light is started. However, as shown by FIG. 11B, the laser light keeps irradiating the same position during the period of the given time T. Thereafter, scanning is started and the laser is turned off at the point of time when the supplied command has reached the target value. However, at that point of time, the laser light is not yet to reach the intended position of irradiation and, hence, the straight line that is actually described by the irradiation with laser light has a length L' which is shorter than the specified length L.
Thus, the conventional laser marking devices have had the disadvantage of being incapable of describing line elements to the correct length.
FIGS. 13A and 13B are comparable to FIGS. 11A and 11B, except that the acceleration and deceleration that accompany the transition of the scanners to a stop mode are taken into account. As soon as the scanners are supplied with a scan start command, a laser ON command is issued to start the application of laser light; however, as FIG. 13B shows, the laser scan speed is slow during the acceleration period and the straight line that is actually described by the irradiation with laser light has a length L' which is shorter than the specified length L.
Consider next the case of describing a geometric form or a character such as "A" (see FIG. 20A). The coordinate data on the locus indicated by a dashed line are computed as the path of scanning with laser light and the X-axis scanner 26 and the Y-axis scanner 27 are controlled for deflection on the basis of those coordinate data. However, as mentioned in the preceding paragraphs, the X-axis scanner 26 and the Y-axis scanner 27 will experience the delay in action on account of inertial force and the like. This has a significant effect in the case where the position signaled to the scanner varies as shown in FIGS. 19A and 19B; if the change in the signaled position is gradual as shown in FIG. 19A, the position of response will reach the target signaled position although there is some delay in the response. On the other hand, if the change in the signaled position is abrupt as shown in FIG. 19B, the position of response will no longer be capable of reaching the target position since the signaled position will change before the position of response reaches the target signaled position.
Referring to the case of describing the character illustrated in FIG. 20A, the signaled position will change abruptly at each position where the locus indicated by dashed line kinks through an acute angle and, hence, the position of response or the position being irradiated with laser light will not be capable of scanning correctly the acute-angled areas as indicated by solid lines in FIG. 20B and the corners of the letter will become round to produce a deformed character.